已知等比数列{an}满足a3=12,a8=3/8,记其前n项和为Sn. (1)求数列{an}的通项公式an; (2)若Sn=93,求n.

问题描述:

已知等比数列{an}满足a3=12,a8=

3
8
,记其前n项和为Sn
(1)求数列{an}的通项公式an
(2)若Sn=93,求n.

(1)∵等比数列{an}满足a3=12,a8=

3
8

a1q2=12
a1q7
3
8
,解得q=
1
2
,a1=48,
ana1qn−1=48×(
1
2
n-1
(2)Sn=48×(
1
2
n-1=93,
(
1
2
)
n−1
=
31
16

解得n-1=log
1
2
31
16

∴n=log
1
2
31
16
+1.